Aqueous formulations of therapeutic proteins (e.g., antibodies) are susceptible to degradation through a number of different mechanisms and as a result of several types of stress conditions. In general, degradation of a therapeutic protein formulation occurs when the protein structure is altered slightly from its fully folded conformation (partial unfolding) exposing hydrophobic residues that interact with an adjacent protein molecule in solution forming an irreversible association. Certain stress conditions such as agitation, freeze/thaw and increased temperature can induce greater protein unfolding leading to accelerated aggregation of the protein and degradation of the protein formulation. Degradation of the protein formulation can be manifested by protein denaturation, the formation of visible particles, the formation of aggregates, the formation of subvisible particles, opalescence of the formulation, loss of biological activity, loss of percent monomer, loss of yield during production and purification, and the like. Exposure of the protein formulation to a liquid/air or liquid/solid interface, such as in agitation or freeze/thaw conditions, allows for a portion of the protein to unfold because of the lack of water at the interface to stabilize the folded structure through hydrogen bonding and hydrophobic effects. Other mechanisms leading to protein degradation include oxidation, hydrolysis, proteolysis, photodegradation, and microbial degradation. It would be desirable to provide a therapeutic protein formulation with improved stability to make the therapeutic proteins more resistant to the stress conditions encountered during their distribution and storage. For example, formulations of therapeutic proteins can encounter stress conditions like freeze/thaw cycles, agitation, long term storage, pumping, filtration, or unrefrigerated storage, where improvements to stability would be advantageous.
In conventional protein formulations, a small amount of a nonionic surfactant, typically Polysorbate 80 or Polysorbate 20, is added to compete with the protein for interfacial surfaces to reduce protein degradation that occurs with its exposure to such surfaces. However, polysorbates themselves can degrade, either through hydrolysis or oxidation, and the resulting degradation products promote aggregation and/or reduce solubility of the protein and destabilize protein formulations. Polysorbates also pose a problem during the manufacturing process of protein therapeutics because of their tendency to form micelles. The formation of micelles can prevent some of the polysorbate from passing through filters such as during an ultrafiltration/diafiltration unit operation, causing a significantly larger polysorbate concentration in the drug substance than intended. For these reasons it is desirable to have a protein formulation that minimizes or is substantially free from conventional surfactants such as Polysorbate 80 and Polysorbate 20.